In recent years, portable terminals used for mobile communication have been increasingly reduced in size and lightened in weight. Also with this, the portable terminals are being made multi-banded to be able to handle a plurality of communication systems and are being given more functions. Therefore, the built-in circuits have increased. For this reason, as the electronic parts used, for improvement of their mounting density, surface mountable small-size parts are being strongly demanded.
One of the key parts of a portable terminal is a surface acoustic wave device. A surface acoustic wave device is provided with a hollow portion to secure a vibration space in the vicinity of an electrode surface where a surface acoustic wave is excited. This vibration space has to be sealed air-tight. In such surface acoustic wave devices as well, together with low loss and an out-of-band cutoff characteristic, surface mountable small-sized surface acoustic wave devices are being demanded.
In response to such demands for smaller size, a surface acoustic wave device with a mounting structure of a type securing a vibration space of the surface acoustic wave by forming a protective cover so as to surround an IDT (inter digital transducer) formed on a piezoelectric substrate and exciting the surface acoustic wave has been proposed (see, for example, Patent Citation 1 and Patent Citation 2).
FIG. 8 is a cross-sectional view of principal parts showing a surface acoustic wave device disclosed in Patent Citation 1 and Patent Citation 2. As shown in FIG. 8, an IDT 52 of a surface acoustic wave element formed on a piezoelectric substrate 51 is surrounded at its periphery by a frame member 54. A lid member 55 is joined to the top of the frame member 54 to thereby form a protective cover 56. Here, 57 indicates a vibration space of the surface acoustic wave, and 53 indicates a connecting line of the IDT 52 and an external circuit. As materials of the frame member 54 and lid member 55, a film-shaped photoresist, resin sheet, etc. have been proposed.
Patent Citation 1: Japanese Patent Publication (A) No. 9-246905
Patent Citation 2: Japanese Patent Publication (A) No. 10-270975